Turbines generally comprise a turbine wheel mounted in a turbine chamber, an inlet passage extending radially inwards towards the turbine chamber, an inlet chamber arranged around the radially outer end of the inlet passage, and an outlet passage extending axially from the turbine chamber. The passages and chamber communicate such that pressurized gas admitted to the inlet chamber flows through the inlet passage to the outlet passage via the turbine chamber, thereby driving the turbine wheel. In the case of a turbocharger for an internal combustion engine, the turbine wheel drives a shaft which in turn drives a rotary compressor.
In one known variable geometry turbine, one wall of the inlet passage is effectively displaceable relative to the facing wall of the inlet passage so as to enable the effective width of the inlet passage to be adjusted. The moveable wall is defined by an annular member generally referred to as a nozzle ring which term will be used below. The position of the nozzle ring is controlled by an actuator mechanism which may be for example hydraulic or pneumatic, the actuation mechanism responding to a control input that is generated in dependence upon various engine operating parameters. One parameter which is used to control the nozzle ring actuating mechanism is the exhaust manifold pressure of the engine to which the turbine is connected. It is useful to be able to arrange for the turbine to respond to exhaust gas pressure fluctuations for example during rapid acceleration, sudden load application, or during engine braking.
It is conventional test-bed practice to measure engine exhaust manifold pressure directly from the engine manifold, and to produce a mean pressure value by smoothing out the pressure fluctuations which result from engine operation. The techniques used are not however suitable for day-to-day use in commercial applications either in terms of cost or sensor durability. Accordingly, although it is known to be desirable to control the variable geometry mechanism of a turbine in dependence upon engine exhaust pressure, in practice this has not been achieved in normal commercial applications.
U.S. Pat. No. 5,522,697 describes a variable geometry turbine in which the turbine comprises a housing, an annular exhaust gas inlet passage defined between walls of the housing, a nozzle ring which is displaceable across the inlet passage, and a control means for controlling the displacement of the nozzle ring in response to variations in sensed parameters. The nozzle ring extends into an annular recess defined by the housing in one side wall of the inlet passage such that a chamber is defined within the recess between the housing and the side of the nozzle ring remote from the inlet passage. The nozzle ring is apertured such that the pressure in the chamber defined between the housing and the nozzle ring is not substantially different from the pressure within the inlet passage. It is indicated in the above U.S. Patent that it is desirable to substantially equalize the pressure within the inlet passage and behind the nozzle ring to minimize the load applied to the nozzle ring displacement mechanism. No suggestion is made however that the pressure within the chamber behind the nozzle ring can be used as a control parameter for the displacement mechanism.